In recent years, a bottlenecking of communication networks has occurred in the portion of the network known as the access network. Bandwidth on longhaul optical networks has increased sharply through new technologies such as wavelength division multiplexing (WDM) and transmission of traffic at greater bit rates. Metropolitan-area networks have also seen a dramatic increase in bandwidth. However, the access network, also known as the last mile of the communications infrastructure connecting a carrier's central office to a residential or commercial customer site, has not seen as great of an increase in affordable bandwidth. The access network thus presently acts as the bottleneck of communication networks, such as the internet.
Power-splitting passive optical networks (PSPONs) offer one solution to the bottleneck issue. PSPONs refer to typical access networks in which an optical line terminal (OLT) at the carrier's central office transmits traffic over one or two downstream wavelengths for broadcast via a remote node (RN) to optical network units (ONUs). In the upstream direction, ONUs typically time-share transmission of traffic in one wavelength. An ONU refers to a form of access node that converts optical signals transmitted via fiber to electrical signals that can be transmitted to individual subscribers and vice versa.
PSPONs address the bottleneck issue by providing greater bandwidth at the access network than typical access networks. For example, networks such as digital subscriber line (DSL) networks that transmit traffic over copper telephone wires typically transmit at a rate between approximately 144 kilobits per second (Kb/s) and 1.5 megabits per second (Mb/s). Conversely, Broadband PONs (BPONs), which are example PSPONs, are currently being deployed to provide hundreds of megabits per second capacity shared by thirty-two users. Gigabit PONs (GPONs), another example of a PSPON, typically operate at speeds of up to 2.5 gigabits per second (Gb/s) by using more powerful transmitters, providing even greater bandwidth. Other PSPONs include, for example, asynchronous transfer mode PONs (APONs) and gigabit Ethernet PONs (GEPONs).
One current limitation of typical PSPONs is their limited reach. Reach generally refers to the maximum distance between the OLT and an ONU in a PON at which the OLT and the ONU can still communicate adequately. Since ONU transmitters are typically weaker than OLT transmitters, the limiting factor in extending reach in a PON has primarily been in the upstream direction and not in the downstream direction. Many network operators desire a solution for extending reach in the upstream direction in a PON that can maintain the ratio of ONUs per OLT.
Some solutions that have been proposed to extend the reach in the upstream direction are to replace ONU transmitters with stronger transmitters, to add a more sensitive receiver at the OLT, or to use amplifiers to amplify upstream signals. These solutions have not been particularly persuasive in the marketplace. Cost considerations have dissuaded many operators from implementing stronger ONU transmitters or a more sensitive receiver at the OLT. Also, operators have viewed amplifiers as requiring costly maintenance and as creating a greater number of points of failure in a PON, decreasing the attractiveness of such an option.
Yet another solution, a wavelength division multiplexing PON (WDMPON), would extend reach in the upstream (and downstream) direction. WDMPONs refer to access networks in which each ONU receives and transmits traffic over a dedicated downstream and upstream wavelength, respectively. In addition, each ONU is “colorless,” meaning that it is interchangeable with any other ONU in any location in the PON. The power loss experienced by a signal in the upstream direction in a WDMPON is much less than in a PSPON, thereby extending reach in the upstream direction. Although WDMPONs would extend reach in the upstream direction, they would do so at a prohibitively high cost for many operators and would provide reach far exceeding current or near-future demand.
Because demand for greater reach in the upstream direction continues to grow (but not at a rate to justify adoption of WDMPONs in most cases), a need exists for cost-efficient solutions to extend the reach in PONs.